(a) Field of the Invention
The present invention relates to organic light emitting devices for display applications and to methods for fabricating such devices.
(b) Background Art
Organic light-emitting devices (OLEDs) are typically manufactured as a sequence of layers deposited on top of each other to form a layer structure. The layer structure typically comprises a first electrode on a supporting substrate and several organic layers disposed between the first electrode and a second electrode. Light output is generated by charge injection into the organic material via the electrodes. The organic material emits photons on excitation by the injected charge. At least one of the electrodes is typically formed from a light transmissive material such as Indium Tin Oxide (ITO) or a thin metal to permit passage of light out of the device.
In accordance with the present invention, there is now provided an organic light emitting device having a layer structure comprising: a first electrode layer; a second electrode layer parallel to the first electrode layer; an electrically conductive and light transmissive layer parallel to the second electrode layer; an electrically insulating layer disposed between the first and second electrode layers; a layer of organic material disposed between the second electrode layer and the conductive layer; an aperture in the organic layer providing an electrical connection path between the conductive layer and one of the first and second electrode layers.
Preferably, the first electrode layer, the second electrode layer, the insulating layer, and the conductive layer each comprise an array of parallel strips, the strip of the first electrode layer extending orthogonal to the strips of the second electrode layer. The strips of the insulating layer and the conductive layer may extend orthogonal to the strips of the first electrode layer. Alternatively, the strips of the insulating layer and the conductive layer may extend orthogonal to the strips of the second electrode layer.
In preferred embodiments of the present invention, there is provided a plurality of apertures in the organic layer each communicating with one of the first and second electrode layers. Each aperture may be located at a different intersection of a strip of the first electrode layer and a strip of the second electrode layer. Each aperture may extend in a direction parallel to the strips of the second electrode layer. Alternatively, each aperture may extend in a direction parallel to the strips of the first electrode layer.
The strips of the conductive layer may be electrically connected to corresponding strips of the second electrode layer. Alternatively, the strips of the conductive layer may be electrically connected to corresponding strips of the first electrode layer.
Viewing the present invention from another aspect, there is now provided a method for fabricating an organic light emitting device, the method comprising: depositing a first electrode layer on a substrate; depositing an electrically insulating layer on the first electrode layer; depositing a second electrode layer on the insulating layer; depositing an organic layer on the second electrode layer; forming an aperture in the organic layer; depositing a light transmissive electrically conductive layer on the organic layer; and forming an electrical connection between the conductive layer and one of the first and second electrode layers via the aperture.
In a preferred embodiment of the present invention to be described shortly, there is provided a passive matrix OLED comprising a substrate on which is disposed a first electrode layer comprising an array of parallel strips. An insulating layer is disposed on the first electrode layer. The insulating layer also comprises an array of parallel strips. The strips of the insulating layer extend in a direction which is orthogonal to the strips of the first electrode layer. A second electrode layer is disposed on the insulating layer. The second electrode layer also comprises an array of parallel strips running orthogonal to the strips of the first electrode layer. The strips of the second electrode layer overlay the insulating layer. The first electrode layer is thus electrically isolated from the second electrode layer by the insulating layer. An layer of organic material is disposed on the second electrode layer. The organic layer extends homogeneously across the strips of the second electrode layer, the first electrode layer, and the intervening insulating layer. Apertures are formed in the organic layer. The apertures communicate with the underlying second electrode layer. The organic layer is covered with a light transmissive, electrically conductive layer. Electrical contacts between the strips of the second electrode layer and the conducting layer are formed via the apertures in the organic layer. The conducting layer comprises an array of parallel strips running parallel to the strips of the second electrode layer. A partition runs between adjacent strips of the conductive layer. The OLED is thus divided into a matrix of addressable light emitting picture elements.
OLEDs embodying the present invention are advantageous in that the first and second electrodes need not be fabricated from a transparent conductor such as Indium Tin Oxide. Instead, the first and second electrode may be selected from a broader range of materials. The electrical characteristics of the first and second electrodes can thus be optimized in the interests of, for example, improving luminous efficiency and reducing potential drops across the display area. Such potential drops would otherwise impair the quality of the displayed image.